1. Field of the Invention
The present invention relates to compound papermaking fabrics. More specifically, the present invention relates to forming fabrics used in the forming section of a papermaking machine.
2. Description of the Related Art
In the art of papermaking, multiple steps occur from the introduction of a pulp slurry to the output of a finished paper product. The initial introduction of the slurry is at the portion of a papermaking machine known as the wet end. Here, the slurry, or fiber suspension, is initially dewatered when the slurry is introduced onto a moving forming fabric, in the forming section of the papermaking machine. Varying amounts of water is removed from the slurry through the forming fabric, resulting in the formation of a fibrous web on the surface of the forming fabric.
Forming fabrics address not only the dewatering of the slurry, but also the sheet formation, and therefore the sheet quality, resulting from the formation of the fibrous web. More specifically, the forming fabric must simultaneously control the rate of drainage while preventing fiber and other solid components contained in the slurry from passing through the fabric with the water. The role of the forming fabric also includes conveyance of the fibrous web to the press section of the papermaking machine.
Additionally, if the drainage occurs too rapidly or too slowly, the quality of the fibrous web is reduced, and overall machine production efficiency is reduced. Controlling drainage by way of fabric void volume is one of the fabric design criteria.
Forming fabrics have been produced to meet the needs and requirements of the various papermaking machines for the various paper grades being manufactured. As the need arises to increase production speed of the papermaking machines and the quality of the paper being produced, the need for improved paper machine clothing allowing for increase production rates and improved quality has resulted.
A twill pattern in woven fabrics is where a fabric is woven with a pattern of diagonal ribs. The twill is typically made by passing the weft threads over one warp thread and then under two or more warp threads. In this manner, in a twill weave, each warp or filling yarn floats across two or more filling or warp yarns with a progression of interlacings by one to the right or left, which forms a distinct diagonal line, or wale. A float is defined as the portion of yarn that crosses over two or more yarns from the opposite direction.
Twill weaves are generally designated as a fraction or ratio, for example 2:1, where the numerator indicates the number of harnesses that are raised, and the denominator indicates the number of harnesses that are lowered.
A straight twill used in forming fabrics is well known, for example, in FIG. 1 a fabric 10 has a straight twill pattern 14. This fabric is prone to guide poorly and can lead to curling of the fabric edges. The twill pattern 14 can also “strike through,” or cause marking, and can cause drainage, or hydraulic marks on the web being formed on the fabric. Additionally, twill patterns also have the possibility of creating holes on the back of the fabric that are too large, resulting in drainage marks.
U.S. Pat. No. 5,152,326 (Vohringer '326) discloses a composite forming fabric having pairs of fabric borne warp binder yarns. However, Vohringer '326 does not prevent diagonal marking of the web caused by the diagonals present in the weave pattern, and unbroken diagonals are not addressed.
U.S. Pat. No. 5,544,678 (Barrett '678) discloses different float lengths achieved by using additional or intrinsic binders.
WO 2004/085740 to Ward discloses the use of varying the warp or weft ratios between the top and bottom layers in order to break up the twill pattern. This break up of the twill pattern occurs only on the paper side surface of the fabric.